The present invention relates to plants for transferring liquid metal from an upstream container to a downstream container, comprising: an upstream container; a downstream container; a tapping spout; a flow regulator for regulating the flow of liquid metal through the taphole; a set of refractory assemblies which are placed between the upstream container and the downstream container, delimiting the tapping spout via which the liquid metal flows from the upstream container into the downstream container, each refractory assembly of the tapping spout having at least one mating surface forming a joint with a corresponding surface of an adjacent refractory assembly; a shroud channel placed around the tapping spout near at least one mating surface between refractory assemblies.
Refractory assembly is understood to mean a monolithic component consisting of one or more types of refractory, possibly comprising other constituents, for example a metal shell. Flow regulator is understood to mean any type of device used in this technical field such as a stopper rod, a slide gate valve, and also a simple restriction.
In a plant of this type, the presence of a flow regulator in the tapping spout means that, when the liquid metal is flowing, there is a pressure drop. If the tapping spout is not perfectly sealed, air can be drawn into it because of this reduced pressure. This is generally the case, in particular at the mating surfaces between the various refractory assemblies which form the tapping spout, the sealing of which is difficult to achieve and to maintain. Air is therefore drawn in, which results in a degradation in the quality of the metal.
In order to solve this problem, it is known to create, by means of a shroud channel, an overpressure of an inert gas around the tapping spout, near each critical mating surface. Inert gas is understood to mean here a gas which does not impair the quality of the tapped metal. Among the gases normally used may be found rare gases, such as argon, but also other gases such as nitrogen or carbon dioxide.
According to a known embodiment, a groove is formed in at least one of the mating surfaces between two adjacent refractory assemblies. This groove is fed with pressurized inert gas and thus forms a closed annular shroud channel placed surrounding the tapping spout. Such an embodiment is known, for example, from U.S. Pat. No. 4,555,050 or EP 0,048,641.
In the particular case in which successive refractory assemblies are able to move with respect to each other, the use of a shroud channel is also known. French Patent Application FR 74/14636 describes a slide gate valve having two plates, each plate having a hole through which the liquid metal passes, the sliding of one plate with respect to the other enabling the flow of liquid metal to be regulated. These two plates each have, along their common mating plane, a U-shaped groove placed head to tail with respect to the other groove so that the arms of one of the Us overlap the arms of the other U, and thus produce a closed annular shroud channel whatever the relative position of the two plates.
According to another known construction, a closed chamber is provided which surrounds the outer part of the mating surfaces, and the chamber is fed with pressurized inert gas. Such a construction is known, for example, from U.S. Pat. No. 4,949,885.
All these known arrangements are used to replace the induction of air by the induction of inert gas, thereby eliminating the chemical problem associated with the liquid metal coming into contact with air.
However, these known solutions have several disadvantages.
The intake of gas into the tapping spout is not eliminated. It is even increased because the groove or the chamber is at an overpressure. This is a drawback particularly in the case of transfer of metal between a tundish and a continuous-casting mould.
The gas taken into the tapping spout ends up in the mould and causes perturbations therein, such as turbulence, movement of the coverage powder and the trapping of this powder in the liquid metal. The gas entrained into the mould may furthermore become dissolved in the liquid metal and subsequently create defects in the solidified metal.
In addition, in order to reduce the speed of the liquid metal as it enters the mould, and thus to reduce the turbulence in the mould, many jet shroud tubes have an outlet cross-section greater than their inlet cross-section.
The speed of flow of the liquid metal then decreases gradually. The presence of a significant quantity of gas in the tube may prevent correct operation of this type of tube: the flow may separate from the walls of the tube and the liquid metal therefore drops as a jet into the mould.
The quality of a mating surface between two refractory assemblies may vary in an uncertain way while the tapping spout is being used. Defects may appear. In particular, in the case of refractory assemblies which can move with respect to each other, wear of the mating surface may lead to significant leakage. Among plants having movable refractory assemblies may be found regulating slide gate valves and devices for changing a jet shroud tube.
One possibility, in order to limit the intake of gas into the tapping spout, is to regulate the flow of inert gas injected into the shroud channel. In this case, if the sealing defect becomes significant, it may happen that the flow rate of inert gas is no longer high enough for only the inert gas to enter the tapping spout. In this case, the pressure in the shroud channel becomes negative and ambient air can be drawn into the tapping spout. On the other hand, if the sealing is good, a fixed flow of inert gas is nevertheless injected into the shroud channel, the pressure therein increases and the inert gas enters the tapping spout without this really being necessary.
Another possibility is to regulate the pressure of the inert gas as it is being injected into the shroud channel. In this case, if the sealing defect becomes significant, the flow rate of inert gas being taken into the tapping spout is high, which leads to the defects mentioned above.
In practice, when the leakage rate is high it is necessary to use these two modes of regulation in alternation, even if this means accepting a certain amount of air being drawn in rather than too great an excess of inert gas. Consequently, management of the regulation is complex and necessarily includes compromises between two types of disadvantages.
The inert gas used is generally argon. The use of argon entails a high cost given that the shroud channel must be permanently supplied and that leaks can be considerable. This is particularly true if the shroud channel consists of an external chamber which cannot easily be sealed and which requires a high flow rate of gas in order to maintain an overpressure therein. This drawback is particularly important in applications of continuous tapping between ladle and tundish.
Moreover, refractory wear pieces are known, from French Patent Application FR 85/02625, which make it possible to introduce, in the actual refractory, an impregnation substance which clogs up the pores in the refractory. This technique prevents infiltration of liquid metal into the pores of the refractory. However, it does not solve the problem of making the joints between successive refractory assemblies gas-tight.
The subject of the present invention is specifically a plant for transferring liquid metal which does not have the drawbacks mentioned above.
The subject of the invention is also a method of improving the sealing of the mating surfaces between refractory assemblies during the use of the tapping spout.
The invention relates to a plant for transferring liquid metal, in particular steel, between an upstream container and a downstream container. Such a plant generally comprises a tapping spout via which the liquid metal flows from the upstream container into the downstream container, this spout being delimited by a set of refractory assemblies placed between the two containers. Each refractory assembly of the tapping spout has at least one surface forming a mating surface with a corresponding surface of an adjacent refractory assembly. A flow regulator makes it possible to regulate the flow of liquid metal through the tapping spout. A shroud channel is placed around the tapping spout near at least one mating surface between refractory assemblies. This shroud channel has an inlet capable of allowing materials to enter.